Evolutionary origins of the structures of NADPH-cytochrome P450 reductase (CYPOR) and nNOS reductase domain (nNOS-red) as shown by overlays of the ribbon structures of D. vulgaris flavodoxin (Fld) and spinach ferredoxin-NADP-oxidoreductase (FNR). A, A model of a putative Fld-FNR complex [31]; B, CYPOR structure [36]. The FMN domain, FAD domain, and connecting domain (CD) are marked; C, structure of the nNOS reductase domain (nNOSred) [42] containing regulatory elements, AR, CT and BF shown in red; D, overlay of the structures of Fld, FNR and CYPOR; E, overlay of CYPOR and nNOSred.